This invention relates generally to the field of digitally controlled continuous ink jet printing devices, and in particular to continuous ink jet printers in which the into droplets are selectively deflected by a transverse flow of gas that has been preconditioned with a solvent to minimize ink drying on the printhead. In both technologies, droplets of ink are ejected from nozzles in a printhead toward a print medium.
Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations.
In drop-on-demand ink jet printing, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets as the medium is moved relative to the printhead. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
Conventional drop-on-demand ink jet printers utilize a pressurization actuator to produce the ink jet droplet from the nozzles of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink. This causes a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a pulse of mechanical movement stress in the material, thereby causing an ink droplet to be expelled by a pumping action. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
The second technology, commonly referred to as continuous stream or continuous ink jet printing, uses a pressurized ink source for producing a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. Typically, continuous ink jet printing devices are faster than drop on demand devices and produce higher quality printed images and graphics.
Other methods of continuous ink jet printing employ air flow in the vicinity of ink streams for various purposes. For example, U.S. Pat. No. 3,596,275 issued to Sweet in 1978 discloses the use of both collinear and perpendicular air flow to the droplet flow path to remove the effect of the wake turbulence on the path of succeeding droplets. This work was expanded upon in U.S. Pat. No. 3,972,051 to Lundquist et al., U.S. Pat. No. 4,097,872 to Hendriks et al. and U.S. Pat. No. 4,297,712 to Sturm in regards to the design of aspirators for use in droplet wake minimization. U.S. Pat. No. 4,106,032, to Miura and U.S. Pat. No. 4,728,969 to Le et al. employ a coaxial air flow to assist jetting from a drop-on-demand type head.
One problem associated with ink jet printers in general and such printers employing gas or air flows in particular, is the drying of the ink. Ink drying in the vicinity of the printhead nozzles can lead to spurious droplet trajectories and nozzle clogging. Additionally, the evaporation of the ink solvent from the droplets as they fly through the air can increase the viscosity of the ink captured by the gutter, thereby causing difficulties during the ink recycling operation when the recycled ink is passed through a filter. This last problem becomes particularly difficult if the loss of solvent in the ink is large enough to cause the pigments in the ink to coagulate.
Solvents have been introduced into the regions surrounding nozzles to prevent ink drying. For example, U.S. Pat. No. 4,228,442 to Krull teaches the use of absorbent or wick-like material disposed partly in a liquid ink solvent to evaporate solvent in front of or around the nozzles prevent drying or thickening of the ink at the nozzles. Miura et al discloses the use of humidified air to minimize nozzle clogging in an air assisted, drop on demand, ink jet printhead. However, none of the inventions described are sufficient to address the problems of solvent evaporation due to high-velocity air streams which interact with droplet streams in printers which employ the air streams to direct droplets along different trajectories according to drop volume.
Clearly, there is a need for a means of mitigating the drying effect that a gas flow has on the ink droplet streams in printers which involve gas flow interaction with ink droplets during printer operation. The primary problem is not the drying of ink at the nozzles, since the air flow in such printers is principally removed from the immediate vicinity of the nozzles. Rather, the difficulty is that the drying of droplets along their trajectory toward the ink catcher increases the viscosity to a point that impedes ink recycling and filtration.
The invention is an ink jet printing apparatus that solves or at least ameliorates all of the aforementioned problems associated with the prior art. To this end, the ink jet printing apparatus of the invention comprises an ink droplet forming mechanism for ejecting a stream of ink droplets having a selected one of at least two different volumes, a droplet deflector for producing a flow of gas that interacts with the ink droplet stream to separate ink droplets having different volumes from one another, and a gas flow conditioner for preconditioning with solvent vapor the gas flow produced by the droplet deflector.
Preferably, the ink jet printing apparatus is a continuous stream ink jet printer, and the flow of gas produced by the droplet deflector is oriented transversely to the stream of ink droplets and functions to deflect smaller volume droplets from larger volume droplets. The solvent used in the gas flow conditioner may be water, and the gas flow is preferably a flow of air.
The gas flow conditioner may include a sensor responsive to a solvent concentration level in the gas flow. The conditioner may also include a control circuit connected to the sensor for adjusting a solvent addition rate to the gas flow in order to maintain a selected solvent concentration in the gas flow.
In operation, the solvent concentration in the gas flow is set at a point that substantially prevents an increase in the viscosity of the ink in the droplets. Consequently, the droplets recaptured by the gutter of the printer may be filtered through the recycling mechanism of the printer without clogging the filter or interfering with the recycling operation.